Food container

ABSTRACT

Disclosed is a container for packaging foods and, particularly, refrigerated and frozen foods. The container includes a tray and a lid both of which are polyolefin coated paperboard.

United States Patent [191 Gordon et al.

1 Feb. 4, 1975 1 1 FOOD CONTAINER [75] Inventors: Robert L. Gordon, Monroe; John C.

Siegele, New City, both of NY.

[73] Assignee: International Paper Company, New

York, NY.

22 Filed: Dec. 20, 1972 21 App1.No.:316,664

[52] US. Cl. 229/30, 229/48 T, 229/32 [51] Int. Cl 865d 5/20 [58] Field of Search 229/27, 30, 32, 31 PS,

[56] References Cited UNITED STATES PATENTS 2,214,525 9/1940 De Murguiondo 229/27 2,633,284 3/1953 Moffett et a1 229/2.5 3,119,540 1/1964 Schenk et a1. 229/30 3,316,102 4/1967 Doll et a1 229/31 FS 3,565,324 2/1971 Odenhagen 229/32 Primary ExaminerWilliam 1. Price Assistant Examiner-Douglas B. Farrow Attorney, Agent, or Firm-Alfred L. Michaelsen [57] ABSTRACT Disclosed is a container for packaging foods and, particularly, refrigerated and frozen foods. The container includes a tray and a lid both of which are polyolefin coated paperboard.

14 Claims, 5 Drawing Figures PATENTED H58 9 5 sum 10F 3 F/GZ FOOD CONTAINER BACKGROUND OF THE INVENTION 1. Field to Which the Invention Pertains The invention disclosed herein pertains to the food packaging art.

More particularly, an object of our invention is to satisfy the demand for a low cost, disposable container for packaging foods wherein the container is so constructed that it will protect the food packaged therein during refrigeration and, additionally, may be used during the heating and serving of the packaged food. The demand for such a container originates from a number of environments wherein large quantities of discreet portions of food must be refrigerated for a substantial length of time and, subsequently, prepared and served within a minimum length of time. Two readily apparent environments which demand such a food package are airlines and educational institutions. In either of these environments, discreet, premeasured portions of food must be stored under refrigerated conditions for extended periods of time, quickly reheated and served. Thus, a food container used in either of these environments must be low in cost, disposable and adapted to protect the food packaged therein under conditions of temperature extremes. Of course, in order to be low in cost, such a container must be of a construction which readily permits the use of high speed forming machines for erecting the container. The invention disclosed herein relates particularly to a food packaging container which satisfies the diverse requirements heretofore discussed.

2. Prior Art The prior art reflects an attempt to provide a food packaging container which would satisfy all of the diverse criteria established by users of such containers. However, to our knowledge, the prior art does not disclose a food packaging container which incorporates, in combination, the individual construction elements which characterize our invention.

For example, considering exemplary prior art patents, U.S. Pat. No. 3,550,835 discloses a food package constructed from polypropylene coated cardboard. The construction disclosed in that patent employs a complex corner joint construction wherein a multiplicity of angularly disposed score lines are used to define the corner joint and each corner joint is provided with multi-sided extension tabs. Although an acceptable container can be fabricated from such a blank, the complexity of the blank requires that sophisticated machinery be employed to form a container from the blank and/or the operating speed of such machinery must be limited. Other equally complex corner joint constructions are disclosed in U.S. Pat. Nos. 3,536,248 and 3,565,324.

Although some of the patents mentioned above suggest the use of plastic coated cardboard, the preponderance of the prior art discloses the use of plastic or metal foils as a construction material for food packaging containers. Although metal foils and plastic may indeed be used to form a food packaging container, their use is not without disadvantage. For example, at the present time the preferred method of reheating premeasured food portions is through the use of microwave ovens. However, through the use of such an oven one would ideally desire to heat the food but not the container which packages the food. Thus, the use of metal foils as a food packaging material is disadvantageous in that amicrowave oven would heat both the food and the food container. Additionally, the use of metal food containers would tend to short circuit a microwave oven.

With respect to food containers which are exclusively made of plastic, the material and manufacturing cost thereof is not as attractive as the material and manufacturing cost associated with the use of the material which we employ, i.e. thermoplastic coated paperboard.

Another problem indigenous to many prior art constructions relates to the difficulty of removing the lid from the tray after the food is heated. Typically, this problem is most severe in the case of all plastic containers or thermoplastic coated paperboard containers wherein the lid is heat sealed to the tray. With such constructions, the heat seal between the tray and the lid is so strong as to necessitate the use of a cutting device, e.g. a knife, to open the container or the lid must be pre-weakened, e.g., by perforations, to enable one to remove the lid.

Finally, as heretofore pointed out, a typical environment in which premeasured food packaging containers are employed requires that the containers are so constructed and arranged as to permit the heating of the food while in the container. In this connection, the prior art has recognized that a situation may be encountered wherein the packaged food, upon heating, will generate gases and cause an increase of the internal pressure within the container. Obviously, if this internal pressure is not relieved, the container will be unattractively deformed and may burst. To avoid either of these undesirable results, the prior art has resorted to a number of construction expedients, almost all of which may be characterized by the fact that they increase either, or both, the material cost or the manufacturing cost of the container. For example, U.S. Pat. No. 2,633,284 discloses a food container construction wherein meltable plugs are provided. Thus, upon heating, the meltable plugs provide a venting means whereby internal pressures may be relieved. It will be appreciated that although the mechanism of providing such meltable plugs essentially solves the problem of relieving internally generated pressures, manufacturing expense is occurred in that the container must be specifically constructed to receive such plugs, the meltable plugs must be accurately positioned on or within the container and the sealing of the container must insure that the meltable plugs are tightly gripped.

ln summary, the prior art discloses a number of containers which satisfy one or a few of the criteria established by users of food packaging containers. However, the prior art does not disclose a container which satisfies all of the criteria previously discussed.

SUMMARY OF THE INVENTION The food packaging container of the invention disclosed herein includes a tray and a lid both of which are constructed of paperboard having a thermoplastic coating on at least the interior surface thereof. Preferable thermoplastics are polyethylene and polypropylene. The tray includes a base portion and a plurality of upstanding walls. Preferably, the base portion is rectangular with four divergingly upstanding walls which are foldably connected to the base portion. The upstanding walls are transversely interconnected by corner closures. Each corner closure is foldably connected to an upstanding wall at one end thereof and is overlappingly bonded to the transverse end of the adjacent upstanding wall. Additionally, the fold line and separation line which initially define the corner closures are arranged such that when a tray is formed, a land is provided at the lower corner of each tray.

Each of the upstanding walls is foldably connected to a horizontal panel. The ends of the horizontal panels are abutting so as to form a horizontal, peripheral flange.

The lid is bonded to the peripheral flange. The bond between the flange and the lid is heat sensitive, i.e. the bond material and the aggregate bonding area are selected and adjusted such that the bond between the lid and the flange will fail at a pre-selected temperature and/or because of an increased pressure within the container.

Preferably, both the tray and the lid are coated on both sides with polyethylene. A preferable coating thickness of polyethylene is approximately 1 mil and a preferable basis weight for the paperboard is in the range of I24 288 pounds per thousand square feet. The bond connecting the lid and the flange may be provided by either a pressure sensitive adhesive, a hot melt adhesive or more generally, a dry, heat-activated adhesive.

An alternate construction which embodies our invention provides a compartment tray.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, in perspective, of a tray and a lid which, in combination, embodies our invention.

FIG. 2 is a plan view ofa blank which may be erected to form a tray of the type shown in FIG. 1.

FIG. 3 is a lid which is used in combination with a tray of the type shown in FIG. 1.

FIG. 4 is a view, in perspective, of another embodiment of our invention.

FIG. 5 is a plan view of a blank which may be erected to form the construction shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of our invention is shown in FIG. 1 and includes a tray 9 having a rectangular base portion 10. Upstanding walls l2, l4, l6 and 18 are foldably connected to the base portion 10 at fold lines 22, 24, 26 and 28 respectively, the aforementioned fold lines being more clearly seen in FIG. 2. Preferably, the upstanding walls are divergently disposed, as shown in FIG. 1, rather than vertically disposed. Divergently disposed walls are preferable because the trays may then be nested prior to being filled and sealed. A desirable angle of divergence is approximately l7 away from the vertical.

Referring to FIG. 2, it will be seen that panels 32, 34, 36 and 38 are foldably connected to upstanding walls l2, l4, l6 and 18 by fold lines 42, 44, 46 and 48, respectively. When the blank of FIG. 2 is erected to form a tray as shown in FIG. I, it will be noted that the panels 32, 34, 36 and 38 are horizontally disposed. Moreover, it will be noted that the ends of the aforementioned panels are transversely cut such that, when the blank is erected to form a tray as shown in FIG. I, the ends of said panels are transversely abutting.

Referring again to FIG. 2, it may be noted that corner closures l3, l5, l7 and 19 are provided between the ends of adjacent wall panels. Moreover, it will be noted that each of the corner closures l3, l5, l7 and 19 is foldably connected, along a transverse fold line, to one of the wall panels and is separated from the adjacent wall panel by a cut or severed line. For example. corner closure 13 is foldably connected to wall panel 12 by fold line 51 and is separated from wall panel 14 by the cut 52. Similarly, fold lines 54, 55 and 58 foldably connect closures 15, 17 and 19 to wall panels I6 and 12, respectively. Cut lines 53, 56 and 57 are analogous to cut line 52.

Referring particularly to the fold lines and to the severed lines which define each of the corner closures, it may be observed that in all cases the fold lines originate at an intersection of two other fold lines whereas the associated cut or severed line originates at a point slightly distant from the intersection of the aforementioned fold lines. For example, referring to corner closure 13, it may be noted that fold line 51 originates at the intersection of fold lines 22, 24 and extends outwardly to fold line 42. However, it will be noted that the severed line 52 originates at a point slightly distant from the intersection of the fold lines 22 and 24. The significance of originating the severed line at a point distant from the intersection of the interior fold lines resides in the fact that by using such a construction, a land will automatically be provided when the blank is erected to form a container. Thus, considering corner closure 13 as an example, if the severed line 52 were to originate at the intersection of the fold lines 22 and 24, when the blank was erected to form a tray, there would be an opening or discontinuity at the lower part of the tray, viz. at the point where the severed line 52 contacted the intersection of the fold lines 22 and 24. Thus, by originating the severed line 52 at a point distant from the intersection of the score lines 22 and 24, the corner closures and the upstanding walls adjacent thereto define a land at the bottom of each corner of the tray and thus an opening or discontinuity is avoided at the bottom of each corner of the tray.

In accordance with our invention, the blank shown in FIG. 2 as well as the tray lid, shown in plan view in FIG.

3, are constructed of paperboard. Further, the lid 29 and the blank shown in FIG. 2 are coated, on the surface whjch will be located interiorly of the container, with a thermoplastic coating. Preferably, both the surfaces of both the lid and the blank are coated with a thermoplastic. Experiments conducted upon the occasion of this invention indicate that polyethylene is generally a preferable coating and, when polyethylene is employed, the preferable coating thickness is approxi mately one mil.

By thermoplastic coating at least the interior of both the blank of FIG. 2 and the lid of FIG. 3, a number of benefits are provided. For example, the resulting container is substantially water proof. Additionally, an even more important benefit resides in the fact that the surfaces of the resulting container construction are relevantly gas impermeable. While the full significance of providing such a construction will be pointed out hereinafter, suffice it to say, at this point, that by providing impermeable surfaces, a functionally superior food packaging container may be obtained.

Preferably, the basis weight of the paperboard is in the range of approximately I24 to 288 pounds.

With respect to erecting the blank of FIG. 2 so as to form a tray such as shown in FIG. 1, such erection and assembly may be easily accomplished through the use of high speed forming and packaging machines. In this connection, it isnoteworthy to observe that the contruction of the blank shown in FIG. 2 is particularly amenable to high speed forming. For example, because each of the corner closures is foldably connected to only one of the adjacent end wall panels, the erection time and forming speed may be significantly reduced. indeed, the construction of the blank shown in FIG. 2 is particularly significant in this regard because the end closures are foldably connected to opposed wall panels. Thus, it may be noted that end closures l5 and 17 are foldably connected to the wall panel 16 while end closures l3 and 19 are foldably connected to the wall panel 12. As such, when the blank of FIG. 2 is formed into a container or tray, the walls 14 and 18 may be folded into an upstanding position without a restraint being imposed thereon through a connection to the wall panels 12 and 16 or any of the corner closures l3, l5, l7 and 19. Either after or simultaneous with the folding of the end panels 14 and 18, into their upstanding position, the opposed walls 12 and 16 may be folded and, thereafter, the end closures 13, 15, 17 and 19 positioned exteriorly of the walls 14 and 18. Thus, it will be appreciated that the construction shown in FIG. 2 facilitates the forming of a tray as opposed to many prior art constructions wherein elements corresponding to the corner closures were foldably connected to all of the upstanding walls. Moreover, since each of the corner closures is attached to only one of the adjacent walls, there is no criticality associated with the sequence in which the walls are folded, e.g. they may be folded simultaneously.

With further regard to the formation of a tray as shown in FIG. 1, it may be noted that the corner closures are positioned exteriorly of the upstanding walls. More particularly, the end closures are overlappingly bonded to the upstanding walls which is adjacent to the upstanding wall to which each end closure is foldably attached. Although a number of methods could be utilized to adhere or bond the corner closures to the upstanding walls, a preferred construction results from coating both the interior and the exterior of the tray with a thermoplastic, e.g. polyethylene. In this manner, the corner closures may be overlappingly bonded to the upstanding walls by heat sealing the corner closures to the walls, i.e. by using the exterior and interior thermoplastic coating to achieve a bond. In the event that only the interior of the tray is coated with a thermoplastic, an adhesive may be used to bond the corner closure to the walls.

Considering the tray lid shown in FIG. 3, a continuous band of adhesive 31 is applied to the interior of the lid 29. The function of the continuous band of adhesive 31 is to bond the lid 29 to the peripheral flange on the tray formed by the panels 32, 34, 36 and 38. Although the adhesive may be applied to either the aforementioned peripheral flange or to the interior of the lid as shown in F IG. 3, certain other aspects regarding the adhesive may not be varied. Thus, the bonding area and the particularly adhesive employed must conform to the criteria now to be set forth.

In order to appreciate the significance of the adhesive connection between the tray and the lid, it must be recognized that an important object of our invention is to provide a food packaging container which will protect the food packaged therein during periods of exposure of low temperature (freezer conditions) while nevertheless providing a container which can be used to heat the food. Although not readily apparent, these two objectives are, in fact, conflicting. Thus, if the food is to be adequately protected from conditions of extremely low temperature, it is imperative that the packaging container completely encompass and seal the food packaged therein. Otherwise, the food may be subject to so called freezer burn. Additionally, if such a package does not provide an effective seal, the food packaged therein may be dried. Although the prior art recognized this problem, the typical approach taught by the prior art contemplates the use of, for example, a paperboard container which is coated on both sides with a polyolefin (for example, polyethylene) and a paperboard', polyethylene coated lid which is heat sealed to the container. Although it is true that such a container provides effective protection from the adverse conditions encountered in a low temperature environment, such a container may not readily be used in a reheating oven since the aforementioned heat seal is essentially stronger than the paperboard. Thus, if such a container is placed in a reheating oven, internal pressures generated by heating the food contained therein will cause an increase in the pressure within the container and the container will almost literally explode or, at a minimum, grossly deform before the container fails. Additionally, a polyethylene heat seal bond is so strong as to essentially prevent the subsequent separation of the lid from the tray. ln contradistinction to the prior art, a container constructed in accordance with our invention avoids such difficulties by employing,

what may be referred to as, a heat sensitive adhesive connection between the lid and the flange of the container. As used herein, the phrase heat sensitive adhesive connection is defined to mean an adhesive connection between the lid and the tray which will fail because of increased internal pressures occurring at a preselected elevated temperature. The pre-selected temperature employed is the maximum temperature above which internally generated pressures caused by reheating the food would cause deformation of the container. Typically, the temperature selected will be a temperature in the range of to 350F. At the preselected temperature or at a temperature within a preselected range, the heat sensitive adhesive connection of our invention will fail, venting of the internal pressure will occur and subsequent removal of the lid is facilitated.

Carefully considering the automatic venting which is obtained in accordance with our invention, it will be appreciated that in order to achieve such automatic venting an adhesive must be selected which has a bonding strength within a particular range and, depending upon the bonding strength of the particular adhesive selected, the adhesive pattern must be adjusted. in other words, in order to achieve a heat sensitive adhesive connection between the lid and the tray, the adhesive and the adhesive pattern are selected such that at the pre-selected temperature, the aggregate strength of the overall heat sensitive adhesive connection will be equal to the outwardly directed force exerted on the lid and the tray due to increased internal pressure. Thus, the particular adhesive pattern employed will vary with both the size of the container and the adhesive material which is selected. However, it will be further appreciated that in order to obtain a container in accordance with our invention, still other constraints relating to the adhesive pattern must be satisfied. For example, although the width of the adhesive connection may be varied, it is critical that the adhesive connection is a continuous pattern. Thus, referring to FIG. 3, it will be noted that the adhesive pattern 31 is a continuous pattern located around the periphery of the lid 29. It is important that the adhesive pattern be continuous because, as previously indicated, a continuous adhesive pattern will insure that the food packaged within the container is adequately protected against freezer burn and, additionally, the continuous adhesive pattern will provide a sealed container and thus insure automatic venting at high temperature.

When packaging certain foods and using certain adhesives, an aesthetically desirable design limitation resides in insuring that the interior longitudinal edge of the adhesive pattern is spaced from the fold line which connects the flange to the upstanding walls. For example, referring to FIG. 2, for purposes of illustration, a fragmentary portion 60 of an adhesive pattern is indicated. As may be noted, the interior edge 61 of the fragmentary portion 60 is spaced from the fold line 48. The desirability of insuring that the interior edge of the adhesive pattern does not extend to the fold line of the flange arises from the fact that if the adhesive employed is a pressure sensitive adhesive and the adhesive pattern did, in fact, extend to or past the flange fold line, food packaged within the container may adhere to the lid when the lid was removed.

With respect to the adhesives which may be employed, it has been determined that at least two different types of adhesives may be used, viz. pressure sensitive adhesives or dry, heat activated adhesives, e.g. so called hot melt adhesives. A particular pressure sensitive adhesive which has been found to be acceptable in an aqueous acrylic emulsion having the following characteristics.

solids content 48% densit 8.6 lbs. per gallon viscosity MOO-1,350 cps (Brookfield, C)

peal strength 600 to 640 gms (180 peal at 12" per minute on cellulose acetate/Mylar) viscosity I400 cps (275F) specific gravities 0.92

heat seal range l75-225F softing point I65F (Ball & Rlng) application temperature 275-325F The use of a pressure sensitive adhesive is desirable when the lid is manually affixed to the tray.

Using either of the two adhesives described above, we have found that a heat senstitive adhesive connection may be obtained using a three-eighth inch wide continuous adhesive pattern in connection with a onehalf inch wide flange if the dimensions of the lid are within the range of 5 inches wide by 6 inches long to 7 inches wide by 10 inches long. A food packaging container of these dimensions and constructed in accordance with our invention will have a heat senstive adhesive connection which provides venting in the range of to 200F.

Another adhesive which would provide a heat sensitive adhesive connection is polyvinyl acetate. Indeed, polyvinyl acetate is typical of a class of thermoplastic materials which possess properties that permit their use both as a coating and barrier material as well as an adhesive. Thus, using polyvinyl acetate as an example, the interior of the lid may be coated therewith and, subsequently, such a lid may be heat sealed to an interiorly coated polyethylene tray and a heat sensitive adhesive connection will be the result.

An alternate embodiment of our invention is shown in FIGS. 4 and 5 wherein FIG. 5 illustrates the blank from which the container of FIG. 4 is formed. The utility of the tray shown in FIG. 4 resides in the fact that compartments are intrinsically provided. Thus, by using such a container, food packaged therein may be divided, e.g. vegetables may be placed in one compartment and meat in another compartment.

Considering the tray construction shown in FIGS. 4 and 5 and particularly the construction of the blank shown in FIG. 5, it may be observed that the blank of FIG. 5 is essentially a combination of two blanks of the type shown in FIG. 2, except that the two blanks are integral. Thus, it may be noted that the various panels and score lines of the blank shown in FIG. 2 are correspondingly indicated in FIG. 5 with the suffix a" and b designating the left hand side and right hand side of the blank shown in FIG. 5, the two blanks being joined by the common fold line 84.

Although our invention has hereinbefore been described by reference to a number of illustrative embodiments, it will be appreciated that other container constructions may be devised which are nevertheless within the scope of our invention as defined by the claims appended hereto.

We claim:

1. A container for packaging, storing and heating foods which comprises:

a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including,

i. a base portion ii. a plurality of upstanding walls foldably connected to said base portion,

iii. a plurality of corner closures, each corner closure foldably connected to a respective one of said upstanding walls and overlapping bonded thereto,

iv. horizontally disposed panels each foldably connected to the top of a respective one of said upstanding walls, the end of said panels abutting each other to form a peripheral flange,

b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and

c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically seperated from said tray.

2. The container of claim 1 wherein the exterior surface of said tray and said lid are coated with a thermoplastic.

3. The container of claim 1 wherein said thermoplastic is polypropylene.

4. The container of claim 2 wherein said thermoplastic is polyethylene.

5. The container of claim 3 wherein the thickness of said polyethylene coating is at least approximately 1 mil.

6. The container of claim 3 wherein said heat sensitive adhesive connection is provided by a pressure sensitive heat adhesive.

7. The container of claim 3 wherein said heat sensitive adhesive connection is provided by dry, heat activated adhesive.

8. The container of claim 3 wherein said paperboard has a basic weight in the range of 124 to 288 pounds.

9. The container of claim 1 wherein said thermoplastic coating on said lid is polyvinyl acetate and said thermoplastic coating on said tray is polyethylene and said heat sensitive adhesive connection is provided by a heat seal bond of polyethylene and polyvinyl acetate.

10. A container for packaging, storing and heating foods which comprises:

a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including,

i. a rectangular base portion,

ii. a first pair of opposed, upstanding walls foldably connected to opposed edges of said base portion,

iii. a second pair of opposed upstanding walls foldably connected to opposed edges of said base portions,

iv. two pairs of corner closures, each pair foldably connected to a respective one of said first pair of opposed upstanding walls and overlappingly bonded to the adjacent second pair of opposed upstanding walls adjacent thereto,

v. a horizontally disposed panel foldably connected to the top of each of said upstanding walls, the ends of said panels abutting each other to form a peripheral flange,

b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and

c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically separated from said tray.

11. The container of claim 10 wherein the exterior surface of said tray and said lid are coated with thermoplastic.

l2. The container of claim 11 wherein said thermoplastic is polyethylene.

13. The container of claim 10 wherein the thickness of said polyethylene coating is approximately 1 mil.

14. A container for packaging, storing and heating foods which comprises:

a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including,

i. a first base portion and a second base portion,

ii. a first plurality of upstanding walls foldably connected to said first base portion,

iii. a second plurality of upstanding walls foldably connected to said second base portion, one of said second plurality of upstanding walls foldably connected to one of said first plurality of upstanding walls,

iv. a plurality of corner closures, each corner closure foldably connected to an adjacent upstanding wall and overlappingly bonded to the other adjacent upstanding wall,

v. a horizontally disposed panel foldably connected to the top of each of said upstanding walls except the two upstanding walls which are foldably interconnected, the ends of said panels abutting each other to form a peripheral flange,

b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and

c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically seperated from said tray. 

1. A container for packaging, storing and heating foods which comprises: a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including, i. a base portion ii. a plurality of upstanding walls foldably connected to said base portion, iii. a plurality of corner closures, each corner closure foldably connected to a respective one of said upstanding walls and overlapping bonded thereto, iv. horizontally disposed panels each foldably connected to the top of a respective one of said upstanding walls, the end of said panels abutting each other to form a peripheral flange, b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically seperated from said tray.
 2. The container of claim 1 wherein the exterior surface of said trAy and said lid are coated with a thermoplastic.
 3. The container of claim 1 wherein said thermoplastic is polypropylene.
 4. The container of claim 2 wherein said thermoplastic is polyethylene.
 5. The container of claim 3 wherein the thickness of said polyethylene coating is at least approximately 1 mil.
 6. The container of claim 3 wherein said heat sensitive adhesive connection is provided by a pressure sensitive heat adhesive.
 7. The container of claim 3 wherein said heat sensitive adhesive connection is provided by dry, heat activated adhesive.
 8. The container of claim 3 wherein said paperboard has a basic weight in the range of 124 to 288 pounds.
 9. The container of claim 1 wherein said thermoplastic coating on said lid is polyvinyl acetate and said thermoplastic coating on said tray is polyethylene and said heat sensitive adhesive connection is provided by a heat seal bond of polyethylene and polyvinyl acetate.
 10. A container for packaging, storing and heating foods which comprises: a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including, i. a rectangular base portion, ii. a first pair of opposed, upstanding walls foldably connected to opposed edges of said base portion, iii. a second pair of opposed upstanding walls foldably connected to opposed edges of said base portions, iv. two pairs of corner closures, each pair foldably connected to a respective one of said first pair of opposed upstanding walls and overlappingly bonded to the adjacent second pair of opposed upstanding walls adjacent thereto, v. a horizontally disposed panel foldably connected to the top of each of said upstanding walls, the ends of said panels abutting each other to form a peripheral flange, b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically separated from said tray.
 11. The container of claim 10 wherein the exterior surface of said tray and said lid are coated with thermoplastic.
 12. The container of claim 11 wherein said thermoplastic is polyethylene.
 13. The container of claim 10 wherein the thickness of said polyethylene coating is approximately 1 mil.
 14. A container for packaging, storing and heating foods which comprises: a. a paperboard tray, the interior surface of which is coated with a thermoplastic, said tray including, i. a first base portion and a second base portion, ii. a first plurality of upstanding walls foldably connected to said first base portion, iii. a second plurality of upstanding walls foldably connected to said second base portion, one of said second plurality of upstanding walls foldably connected to one of said first plurality of upstanding walls, iv. a plurality of corner closures, each corner closure foldably connected to an adjacent upstanding wall and overlappingly bonded to the other adjacent upstanding wall, v. a horizontally disposed panel foldably connected to the top of each of said upstanding walls except the two upstanding walls which are foldably interconnected, the ends of said panels abutting each other to form a peripheral flange, b. a lid disposed on said flange, the interior surface of said lid coated with a thermoplastic; and c. a heat sensitive adhesive connection between said lid and said flange wherein said heat sensitive adhesive connection is adapted to fail when the pressure within said container reaches a predetermined value whereby said lid is automatically seperated from said tray. 